In recent years, in addition performing printing by an ink-jet printer on substrate composed of paper, fabric, plastic, ceramic or the like, ink-jet printer is also being used when forming oriented film on transparent glass substrate for equipments such as liquid crystal display equipment, or when coating color filter on transparent glass substrate in organic EL display equipment. Specifically, in the case of liquid crystal display equipment, oriented film material such as transparent PI ink (transparent polyimide ink) or transparent UV ink is discharged and coated on glass substrate (see patent document 1, for example), and in the case of organic EL display equipment, coating material such as, for example, transparent UV ink is discharged and coated on glass substrate.
In such cases, a desirable discharging aspect is such that ink discharges from a plurality of nozzles on a print head of an ink-jet printer are uniform between each nozzle. For example, in the case of a printed material in which color is a main constituent, ink is discharged on coating article having a characteristic that absorbs ink (ink-absorbent characteristic) after taking into consideration on how to produce colors with uniform density from each nozzle. Therefore, the uniformity of color density on the coating article having ink-absorbent characteristic can be easily achieved if the ink droplet discharged from each nozzle is adequately uniform.
However, it is extremely difficult to form a thin film having a uniform thickness on a coating article having a characteristic that does not absorb ink (non ink-absorbent characteristic), for example the glass substrate described above, using conventional ink-jet printer. Therefore, it is not only particularly difficult to form a thin film having a uniform thickness of less than 1 μm on a coating article with such non ink-absorbent characteristic, but even when forming a film having a uniform thickness of 1 μm or more, the discharging aspect of ink droplet from each nozzle is required to be improved precisely.
Attempts had been made to produce an adequately uniform discharging aspect of ink droplet discharged from each nozzle by making fine adjustment on the applied voltage of each nozzle (operating voltage that generates pressure fluctuation in pressure chamber through opening of each nozzle) on a print head of an ink-jet printer, in order to form a film having a uniform film thickness on a coating article. According to patent document 2 below, correction of applied voltage (operating voltage) that adjusts discharging speed of ink droplets from nozzle is disclosed.
Further, when using an ink-jet printer to discharge ink on a coating article having a non ink-absorbent characteristic such as the glass substrate described above, if the spreading of ink droplets after impact is not sufficiently controlled, problems such as the failure of each impacted ink droplets fusing together will occur, hence causing difficulty in forming a good quality film having uniform film thickness on the coating article. Therefore, during the actual forming of film by discharging ink on the coating article having a non ink-absorbent characteristic, it is extremely important to precisely control, in advance, the spreading of the ink droplets after impacting the coating article.
In view of such circumstances, a conventional method for controlling the spreading of ink droplets after impacting a coating article is by dribbling pure water or ink on the surface of the coating article, and thereafter measuring the contact angle α formed by the surface of the periphery 13a of the ink droplet 13 and the surface 12a of the coating article 12 as represented in FIG. 19, using an existing contact angle gauge. Here, the smaller and sharper the contact angle α is, the better hydrophilic property the surface of the coating article possesses and the wider the ink droplet will spread, hence indicating an advantage for forming film having uniform thickness. However, the spreading of the ink droplet does not solely differs according to the characteristic of the ink or the surface aspect of the coating article, but is also influence by other factors such as the size or the discharge rate of the discharged ink droplet. Taking such matters into consideration, it is not difficult to come to a conclusion that the spreading of ink droplet cannot be adequately controlled using the conventional method.
In addition, the forming of oriented film by dribbling oriented film material on the above described glass substrate using ink-jet printer is another know application of ink-jet printer. However, due to the ink-jet method, problems such as nozzle defects, nozzle direction and choking of nozzle by oriented film material occur, and hence causing difficulties in adequately dribbling oriented film material at a required location and in a required quantity.
In order to solve such problem, the droplet state is inspected and oriented film material is added and restored if insufficient, and abandon if unsuitable. As a method for inspecting the droplet state, it is suggested (in patent document 3) to measure the film thickness by passing a substrate, on which an oriented film is formed, through an interference film thickness measuring apparatus; and if it is insufficient, re-dribble oriented film using ink-jet nozzles for reinforcement, and thereafter measuring the film thickness again using the interference film thickness measuring apparatus.
[Patent Document 1] JP No. 2001-42330A
[Patent Document 2] JP No. 2003-191467A
[Patent Document 3] JP No. H9-166783A
In recent years, the increase in size of glass substrate, for example, has created needs to increase the coating width and coating length of the ink by ink-jet printer and to decrease the coating time. Under such circumstances, even by adjusting the applied voltage of each nozzle on the print head, which ought to achieve uniform film thickness as described above, various problems exist in the adjustment itself.
In order to increase the coating width of the ink, the coating width of the print head itself and the number of nozzles need to be increased significantly. This inevitably leads to an extremely expensive print head, and the system that controls the print head will also becomes complicated and expensive. Further, the amount of information of the control system will enormously increase, hence creating problems such as an inappropriate decrease in coating speed of ink from each nozzle.
Moreover, even by respectively adjusting the applied voltage of each nozzle in the existing ink-jet printer to achieve a uniform coating of ink droplets, there is limitation in the precision of applied voltage and achieving sufficient uniformity is difficult. For example, even though the uniformity (variability) of an oriented film formed on a glass substrate used in the above-described liquid crystal display equipment is required to be about 2%, the inherent discharge precision of the ink-jet printer used presently is about 5%. Therefore, in order to adjust the applied voltage of each nozzle in such ink-jet printer to achieve a film uniformity of about 2%, the precision (resolution) of the applied voltage needs to be increased. However, this is virtually impossible to realize, and even if realized, the control system will be further complicated and the amount of information overwhelming, consequently leading to a further decrease in the coating speed of ink.